1. Field of the Invention
This invention relates to crossed-field amplifiers and, more particularly, to a cathode configuration for use in a crossed-field amplifier (CFA) which provides a reduction in noise output from the CFA.
2. Description of Related Art
Crossed-field amplifiers (CFAs) have been used for several years in electronic systems that require high RF power, such as radar systems. A CFA typically includes a central cylindrical shaped cathode coaxially disposed within an annular anode structure with an interaction region provided between the cathode surface and the anode. The anode structure may include a vane network which provides a slow wave path for propagation of an RF input signal. Upon application of an electric field between the cathode and the anode, the cathode surface emits a space-charge cloud of electrons. A magnetic field is provided perpendicular to the electric field, which causes the emitted electrons to spiral into cycloiding paths in orbit around the cathode. When RF fields are present on the slow wave structure, the rotating space-charge cloud is distorted into a spoke-like pattern. Electron current flows through the spokes from the cathode to the anode, with the spoke-like cloud rotating in phase with the phase velocity of the RF signal. The interaction between the electron current and the RF signal causes the signal to become amplified.
One problem typically experienced with CFAs is that of excessive noise resulting in the RF signal output. The noise is believed to be due to out of phase electrons emitted by the cathode which interact with the RF signal. These out-of-phase electrons generally follow deeply cycloiding paths and do not become drawn into the rotating spokes. This noise is also commonly referred to as phase jitter.
Numerous techniques have been applied in an attempt to reduce the undesirable noise level. Such techniques to reduce the CFA noise have included cathode driven CFAs, axial cathode slots, partially non-reentrant CFAs, and altered cathode materials. While some of these techniques yielded slight reduction in noise, each caused undesirable changes in CFA performance characteristics.
Another such technique is disclosed in U.S. Pat. No. 4,814,720, by MacPhail, which is assigned to the common assignee. MacPhail discloses the use of a slow wave structure on the cathode which is aligned with the slow wave structure of the anode. The cathode slow wave structure permits cross-coupling of the RF input from the slow wave structure of the anode, which sharpens the oscillation pattern of the electron beam.
Accordingly, a need exists to reduce the noise output from a crossed-field amplifier, while maintaining acceptable levels of CFA performance.